Radiometers receive infrared radiation from sources in their fields of view and transform the received radiation into electrical signals which can be measured and recorded without physical contact with the target sources being measured. A basic radiometer includes an optical system, a chopper, detector, a reference source, a synchronizing signal generator, and an electronic processing system. The optical system may take many forms but is frequently a mirror telescope consisting of concave primary and convex secondary mirrors with the focusing usually accomplished by moving the secondary mirror along the optical axis. An infrared detector is located at the focal point of the optical system to receive and convert the infrared radiation into a detector signal. A black body radiation reference source is used as a standard against which the target radiation is continuously compared. An optical chopper, for example, in the form of a mirror surface, sector-disc-shutter is driven by a motor and rotates in front of the reference source. The detector alternately senses target radiation as the rotating disc is opened and closed, and the detector output comprises an alternating signal, the peak-to-peak voltage of which is precisely proportional to the difference between the target radiation and the known radiation from the black body reference source. A synchronizing signal generator develops a square wave signal which drives a phase sensitive demodulator in the electronic processing system. As the blades of the optical chopper rotate, the synchronizing signal generator generates a square wave signal of exactly the same frequency as the detector output signal and the synchronizing signal can be moved so as to adjust the phase of the output.
An amplifier located close to the detector amplifies the detector output signal and generally speaking a synchronous rectifier is used driven by the synchronizing signal generator which demodulates the amplifier output producing a DC signal exactly proportional to the difference between the radiation from the target and the known radiation from the reference black body source. The polarity of the demodulated output signal indicates whether the target is hotter or colder than the reference source. Since variations in the output signal are precisely proportional to variations in the target temperature, the DC signal generated by the radiometer can be connected to a control system for the real time scanning of spectral targets and/or temperature monitoring or control of other devices or electronic circuits based on the temperature of the target.
Conventional full wave rectified signals contain a large amount of ripple which must be heavily filtered to reduce demodulation noise, generally cutting system frequency response down to one quarter of the input signal frequency. Also, when the target radiation is small, large variations in noise can produce errors and in some cases can actually swamp out the signals desired to be detected. The problem is caused by the synchronous demodulation technique in which the switching circuit filters provide a lag between readings and are not achieving the full band width of the optical chopper.